Heat exchange system



May 12, 1942.. w. G, NOA-CK' 2,282,740

HEAT EXCHANGE SYSTEM Filed NOV. 7, 1959 Wazla rduswymd Patented May 12, 1942 near sxcnsncs srs'rsM Walter Gustav Noack, Baden, Switzerland, as-

signor to Ak'tiengesellschaft Brown, Boverl &

Cie., Baden, Switzerland Application November '1, 1939, Serial No. seam In Germany November 11 1938.

2 Claims. (Cl. 263-19) This invention relates to a heat exchanger for heating gases under pressure, and more particularly to a blast heater for supplying hotairunder pressure to a blast furnace.

The heat exchanger is of the multi-stage type and preferably is a two stageheat exchanger having an upper stage into which the hot gases from a combustion chamber are delivered and a large input of heat energy must be supplied by the combustion. Thetemperature oi the products of combustion leaving the upper stage oi the heat exchanger under such conditions is toohigh to permit the products oi combustion to be used directly for driving the hot gas turbine because the materials of construction of the turbine will lower stage from which the. spent hot gases are exhausted. The heating gases and the air or the like to be heated flow through the heat exchanger in passages separated by metallic walls,

i. e. in indirect heat-exchange contact, and preferahly in counter-current flow. changer is divided into an upper and a lower stage chiefly for reasons of manufacture. The tubes or heat exchange surfaces of the upper sta e are made 01 expensive highly heat-resistant materials because they are subjected to the highest temperatures.

The combustion chamber for supplying heating gases to the heat exchanger operates under a pressureconside'rably above atmospheric, and the heatinggases which flow through the upper stage oi the heat exchanger are at approximately the pressure of the combustion chamber.

The super-atmospheric pressure in the combustion chamber and upper stage oi the heat The heat exexchanger is established by forcing air for cominto said chamber from an air compressor which is driven byagas turbine. The gas turbine is positioned between the upper and lower stages of the heat exchanger and motivatins gase for driving the turbine are supplied in part by the heating gases from the upper stage and in part. from a source of cooler gas under pressure as will be presently explained. Exhaust gases from the turbine pass into the lower stage of the heat exchangerr Preferably, the gas turbine also drives a'compressor for supplying iuel to the combustion chamber.

The'gas turbine drives not only the iuel compressor and the compressor for supplying comnot stand the high temperature.

In order. to achieve these operating conditions, according to the invention, means is provided for mixing a quantity of cooler air under pressure with the hotter products of combustion passing from the upper stage of the heat exchanger to the gas turbine whereby to produce a turbine motivating gas of intermediate temperature, oi suiiicient' volume and at a. suilicient pressure to drive the gas turbine.

The air to be bled into the turbine motivating gases may be taken'irom either the compressor for the combustion air or irom the compressor for the blast to be heated. It can, however, be v preheated compressed air taken from the tubes of the heat exchanger carrying the air being heated for the blast.

The compressorior the blast to be heated and the compressor for the combustion air advantageously can be combined as a unitary compressor suppiyins both uses.

The invention will be more particularly described by way of illustration with reference to bustion air to thecombustion chamber but also 4 furnishes power in compressing the air to be heated or the hot blast. The air for the hot blast and the combustion air may be supplied by a single air compressor or separate air compressors may be employed to supply each use.

Theoutput 0! the gas turbinehigh in order to drive the several compressors referred to. Also, it is desirable to heat the air for the hot blast to the highest practical temperature in the heat exchanger. To this end, a

ust be very the accompanying drawing in which:

Fig. l. is a atic representation oi a heat exchange system embodying the principles oi the invention; and

Fig. 2 is a ditie representationoi a modified embodiment of the invention.

mm. 1. ilisth'e cpmpressoriortheairblast' tobeheated,-havinganairinletat Ila. are the tubes (or heating surfaces) of the upper land! 'stageA, I those of the lower stage 3 of a heat exchange device for heating an air blast. 4 is thecompressoriorthecombustionairand I is the compressor tor the fuel. The two 1 burned in the combustion chamber i and pass through I to the upper stage A oi the heat exchanger as hot gas and thence through l to the gas turbine i.

-A should not greatly exceed the pressure within thetubes I,I,becauseatthehightemperature obtaining the tubes are not capable of withstanding a high pressure diflerential. v On'the otherhand.theturbine.inviewofthestrengthoithevaneacannotsafelywitmtandthehigh 55. temperatures of the hot gases leaving chamber;

we; are

The pressure within the chamber A but should be supplied with a motive gas at a temperature not above about 550 C. In order to'compress the amounts of air required with motive gas at this temperature, the turbine must be supplied withtan amount of such relatively cooler motive gas which is not substantially less than the total amount of air to be compressed. In order to meet these requirements, a certain amount of air is introduced into the hot gases passing through conduit 8 from the compressor for the blast to be heated, through pipe In, or from the compressor for the combustion air, through pipe II, this added airserving the two purposes of 1) cooling the hot gases to a temperature (e. g., 550 C.) which will not be harmful to the turbine vanes and (2) increasing the volume of the turbine motive fluid to the quantity sufficient to supply the necessary compressing capacity. Since this added air does not have the temperature that it must have as motive fluid for the gas turbine, the temperature of the hot gas leaving at 8 must be maintained somewhat higher and the added air warmed up by the excess heat in the latter.

The motive gases leave the gas turbine at I2, and then enter the lower stage B of the heat exchange device. They pass into the open air at l3 or into an exhaust gas pipe.

Although with this method of operation, it is possible to carry on the combustion in the combustion chamber 6 with sufficiently high air excess that inadmissibly high hot gas temperatures are avoided, it may be advisable for reasons of economy of piping to supply cold blast to the tubes which are first contacted with the hot heathigher than that for large pressures and small volumes, and in addition the temperature in front of the gas turbine can be lower and the pressure on both sides of the walls of the tubes is about equal, so that high temperatures can be used and materials highly resistant to scaling but of lower strength can be employed for the tubes. For such reasons, this method of operation is far superior to the use of exhaustors.

I claim:

1. A hot blast plant comprising, in combination, a multi-stage metallic heat exchanger for heating the blast, said heat exchanger comprising a high temperature stage and a low temperature stage, a combustion chamber for supplying hot combustion gases under pressure to the high temperature stage of said heat exchanger, a gas turbine supplied with hot motive gases from the high temperature stage of the heat exchanger and exhausting into the low temperature stage of the heat exchanger, a compressor driven by said gas turbine for supplying compressed fuel to said combustion chamber, compressor means driven by said gas turbine for supplying compressed air to the low temperature stage of the heat exchanger for the blast to be heated and ing gases, as is shown in Fig. 1. A portion of the air to be heated from the compressor I 4 passes to the group of tubes I in stage A and a portion to the group of. tubes 3 in lower stage B. Both portions come together in the collector IS.

The illustrative embodiment of Fig. 2 diflers from that of Fig. 1 only in that here the compressor for the blast l4 provides both the combustion air and the air to be mixed with the turbine motivating gases. I6 is the branch for the combustion air, ID that for theair to be mixed with the turbine motivating gases. This air is already preheated in stage B. Cold air may,'of course, be taken likewise from pipe I].

In general, the pressure of the motive gas before entrance into the gas turbine will be practically the same as the pressure of the blast, since in this way, as already mentioned, the stress on the tubes in the upper stage will be least.

In Figs. 1 and 2 of the drawing, the combustion chamber is shown separate from the heat exchange device. It may, of course, be combined with the upper stage of the heat exchanger.

The great advantage of increasing the amount 01' motive gas instead of raising the motive gas pressure drop consists, in brief, in the circumstance that the efliciency of the compressor for large volumes and small pressures is very much to the combustion chamber, and means by-passing the combustion chamber and the high temperature stage of the heat exchanger for introducing a portion of the compressed air from said compressor-means into the stream of hot motive gases supplied to the gas turbine from the high temperature stage of the heat exchanger.

2. A hot blast plant comprising, in combination, a multi-stage metallic heat exchanger for heating the blast, said heat exchanger comprising a high temperature stage and a low temperature stage, a combustion chamber for supplying hot combustion gases under pressure to the high temperature stage of said'heat exchanger, a gas turbine supplied with hot motive gases from the high temperature stage of the heat exchanger.

and exhausting into the low temperature stage of the heat exchanger, a compressor driven by said gas turbine for supplying compressed 'fuel to said combustion chamber, an air compressor driven by said gas turbine, means for conveying a portion of the compressed air from mid air compressor to the low temperature stage of said heat exchanger, means for conveying a portion of the heated compressed air from said low temperature stage to said high temperature stage, means for conveying another portion of the heated compressed air from said low temperature stage to said combustion chamber, means for bleeding another portion of the heated compressed air from said low temperature stage into the stream of hot motive gases supplied to said gas turbine, and means for bleeding a portion of the compressed air from said air compressor directly into the stream of hot motive gases supplied to said gas turbine.

WALTER GUSTAV NOACK. 

